This invention relates to minimizing the risetime of a pulsed microwave output signal. More specifically, the preferred embodiment of the invention minimizes the output risetime of a microwave signal generated by a YIG-tuned frequency multiplier (YTM) which has adverse input capacitance tending to degrade the risetime of the generated output. The YTM, for instance, may be the source for the output of a very broadband signal generator, such as one having a range 2 GHz to 26.5 GHz. As part of such a broadband signal generator, the YTM source must meet certain design goals. For example:
1. The output power of the source must be accurately calibrated and metered during pulse, continuous wave, and amplitude modulations; PA1 2. The output power has to be optimal; this is especially important in the higher microwave frequency ranges, e.g., 18 GHz-26.5 GHz, or K-band, where transmission losses are correspondingly greater; and PA1 3. When the microwave signals are pulse modulated, each microwave pulse must have a short risetime, for example, on the order of 50 nanoseconds or less.
In the prior art, a YIG-tuned step recovery diode multiplier is often used for generating high frequency signals and for achieving some of these goals. These high frequency signals are in K-band or higher and are generally pulse modulated. To pulse modulate these signals in the prior art several methods are used; the following approaches represent some of these methods.
Post-multiplication Modulation:
This prior art approach pulse modulates a signal after frequency multiplication. This method is illustrated in FIG. 1. The advantage of this system is that microwave pulses of the modulated signal have a very short risetime, i.e., a risetime on the order of 10 to 15 nanoseconds, typically. Several disadvantages, however, are apparent in this method. One disadvantage is that the pulse modulator must be very broadband; such a modulator is difficult and expensive to design. Additionally, pulse modulation at high frequency, e.g., K-band, is very inefficient, leading to very high power loss when the modulator operates at these frequencies. Furthermore, the pulse modulator presents a varying load impedence to the frequency multiplier, thus introducing distortion to the microwave pulse.
Post-multiplication Modulation and Amplification:
Another system is shown in FIG. 2. This system is identical to the one in FIG. 1 with the addition of an amplifier. This system has a short microwave pulse risetime (10 to 15 nanoseconds) and high power output; however, the amplifier following the pulse modulator is required to operate over the entire frequency range. Because of the difficulties in designing a high frequency broadband amplifier, such an amplifier would be unfeasible. One method used to circumvent the design difficulties of the amplifier is to use a K-band amplifier and electromechanical switches to bypass the amplifier in all frequencies except K-band, viz., 18 GHz-26.5 GHz. In this way, the output signal is amplified in the frequency range most needed. This scheme, however, presents additional difficulties: (a) switching time is slow when switching into K-band; and (b) when a sweeping signal generator is used, the switch may be required to switch many times per second, thus resulting in an early failure of the switch.
Pre-multiplication Modulation:
An alternate system is to pulse modulate the signal before multiplying the frequency. This is shown in FIG. 3. In this system, the only components which must be broadband are the frequency multiplier, the detector, and the coupler. All other microwave components operate over a much narrower frequency range, that is, in the same frequency range as the premultiplied microwave input frequency. Thus, these components are relatively cheap and easy to design. The disadvantage of this system is that the frequency multiplier distorts the microwave pulse. The risetime of the microwave output pulse is on the order of 150 nanoseconds. This microwave pulse distortion and long risetime is primarily caused by the frequency multiplier's input D.C. blocking capacitance being charged to its steady state voltage.
The present invention has been designed particularly to reduce the degradation in risetime caused by capacitance, such as that found in the third modulation method discussed. It has also been designed generally to avoid the disadvantages of the prior art methods.